Autism is a brain-based disorder characterized by significant impairment in communication and emotional understanding/expression in the presence of stereotyped interests and activities. It is typically not diagnosed until 3 years of age. Yet there is evidence that the brain abnormality in autism has its origin in prenatal life or at birth. This project aims to improve outcome in toddlers with autism by facilitating access to earlier intervention and by identifying critical targets for early intervention. Four groups of toddlers will participate in this prospective, longitudinal study of autism: siblings of children with autism, late talkers referred by local professionals (having no family history of autism), toddlers with non-familial autism, and typically developing controls. Assessments of general development, communication, and functions related to interpersonal synchrony (joint attention, contingent imitation, shared positive affect) will be conducted. Our preliminary evidence suggests that, by 18 months of age, autism can be differentiated from language delay based on functions related to interpersonal synchrony (joint attention, shared positive affect). This project will extend existing evidence that a disruption in interpersonal synchrony may differentiate autism from language delay in toddlers. We will test the hypothesis that targeting these interpersonal synchrony goals in early intervention may accelerate language and communication development, and ultimately improve outcomes for children with autism. This project also aims to define early neurobiological mechanisms that may lead to improved medical interventions for autism.